fix out of bounds when nrows == 1

[raphaeldussin]

when using with internal_tides, I encountered a case where layers get merged into one
and create an array out-of-bounds issue. @Hallberg-NOAA suggested this fix and it solved my problem.

[codecov-io]

Codecov Report

Merging #1104 into dev/gfdl will increase coverage by 0.30%.
The diff coverage is 50.87%.

@@             Coverage Diff              @@
##           dev/gfdl    #1104      +/-   ##
============================================
+ Coverage     45.76%   46.06%   +0.30%     
============================================
  Files           215      214       -1     
  Lines         69638    69361     -277     
============================================
+ Hits          31872    31954      +82     
+ Misses        37766    37407     -359     

Impacted Files	Coverage Δ
config_src/solo_driver/MESO_surface_forcing.F90	0.00% <0.00%> (ø)
config_src/solo_driver/MOM_surface_forcing.F90	27.90% <0.00%> (ø)
...nfig_src/solo_driver/Neverland_surface_forcing.F90	0.00% <0.00%> (ø)
config_src/solo_driver/user_surface_forcing.F90	0.00% <0.00%> (ø)
src/core/MOM_PressureForce_analytic_FV.F90	44.18% <ø> (ø)
src/core/MOM_variables.F90	57.77% <ø> (ø)
src/diagnostics/MOM_debugging.F90	13.21% <0.00%> (-0.08%)	⬇️
src/diagnostics/MOM_wave_speed.F90	42.66% <0.00%> (ø)
src/ice_shelf/MOM_ice_shelf.F90	0.00% <0.00%> (ø)
src/ice_shelf/MOM_marine_ice.F90	0.00% <0.00%> (ø)
... and 45 more

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[raphaeldussin]

@marshallward @adcroft this one should be an easy one to review

[marshallward]

Gaea regression test: https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/commits/pr/1104

Test has passed; no regressions.

[Hallberg-NOAA]

I agree that this commit is correct and will fix a clear problem. I approve.